Images that exhibit high contrast are highly desired in the photographic industry because such images appear sharp to the viewer. The industry has thus sought to ways to increase the contrast of the images formed in its products.
Contrast has typically been measured by one of two parameters: the gamma (.gamma.) of the emulsion layers contained in a product, and the sharpness of the toe section of the emulsion layers' D-log E curves. By gamma, it is usually meant the slope of the straight-line section of the D-log E curve of a given emulsion layer, typically determined at the speed point. By sharpness of the toe section, it is usually meant the relative density of the toe section. For instance, a sharp toe corresponds to a relatively low (small) toe density, and a soft toe corresponds to a relatively high (large) toe density. Generally, the point at which toe density is measured corresponds to 0.3 log E fast of the speed point (typically the point on the D-log E curve where density equals 1.0), although toe density may properly be measured at any point prior to the curve's primary increase in slope.
If either the value of gamma is high or the toe is sharp, then the image has a relatively high contrast. If the value of is low or the toe is soft, the image has a relatively low contrast.
In an effort to improve the contrast and other photographic properties of photographic elements based on silver halide emulsions, the photographic industry has incorporated various compounds into photographic elements. These compounds are of two general types: dopants and grain surface modifiers. Dopants are substances added to an emulsion during silver halide precipitation which become incorporated within the internal structure of the silver halide grains. Because they are internally incorporated, they are distinguished from substances added post-precipitation such as chemical or spectral sensitizers. These latter compounds are externally associated with the surface of the silver halide grains and are thus more properly referred to as addenda or grain surface modifiers.
Depending on the level and location of dopants, they may modify the photographic properties of the grains. When the dopants are transition metals which form a part of a coordination complex, such as a hexacoordination complex or a tetracoordination complex, the ligands can also be occluded within the grains, and they too may modify the grain's photographic properties.
Specific examples of doped silver halide emulsions can be found in U.S. Pat. No. 4,147,542, which discloses the use of iron complexes having cyanide ligands; U.S. Pat. Nos. 4,945,035 and 4,937,180 which disclose the use of hexacoordination complexes of rhenium, ruthenium and osmium with at least four cyanide ligands; and U.S. Pat. No. 4,828,962, which discloses the use of ruthenium and iridium ions to reduce high intensity reciprocity failure (HIRF).
Recently, dopants have been described which comprise transition metal complexes having nitrosyl or thionitrosyl ligands. European Patent Applications 0325235 and 0457298 disclose the use of one such complex, namely potassium ferric pentacyanonitrosyl. A second type of dopant, rhenium nitrosyl or rhenium thionitrosyl is disclosed in U.S. Pat. No. 4,835,093; and a third, dicesium pentachloronitrosyl osmate, is disclosed in U.S. Pat. No. 4,933,272.
It has also been known to use combinations of dopants in silver halide emulsions. Such combinations of dopants can be found in U.S. Pat. No. 3,901,713, which discloses the addition of both rhodium and iridium compounds during emulsification or the first ripening; U.S. Pat. No. 3,672,901, which teaches the combined use of iron compounds and iridium or rhodium salts.
Methods of improving the photographic characteristics of silver halide emulsions have also consisted of adding transition metals to the emulsions during chemical or spectral sensitization. As mentioned, transition metals added in this manner, because they are added subsequent to silver halide precipitation, are referred to as grain surface modifiers rather than dopants.
The most prevalent chemical sensitizers are the gold and sulfur sensitizers. Other chemical sensitizers are platinum and iridium salts, and complexes of rhodium, osmium, and ruthenium.
Combinations of grain surface modifiers and dopants, or of two grain surface modifiers, have been utilized and are disclosed in, for example, U.S. Pat. Nos. 5,252,451 and 5,256,530.
Although it is known to employ transition metals, and combinations thereof, as either dopants or grain surface modifiers, prior applications of such transition metals have yielded emulsions exhibiting inferior contrast improvement. This has often been the result of one dopant or grain surface modifier exerting an insufficient effect; or the result of a combination of dopants or grain surface modifiers exerting opposing effects.
Accordingly, it would be desirable to overcome these deficiencies by providing a high contrast silver halide emulsion exhibiting a high gamma and/or sharpened toe, wherein a combination of transition metal complexes imparts the high contrast characteristic.